There are many types of voltage amplifiers, but all share similar characteristics and similar limitations. For illustrative purposes, FIG. 1 shows the internal structure of a basic amplifier 11. A typical amplifier 11 has an input signal V.sub.IN at an input node 15 and an output signal V.sub.OUT at an output node 17.
V.sub.OUT is a function of V.sub.IN determined by the internal structure of amplifier 11. In the present example, input signal V.sub.IN is internally coupled to the control gate of an nmos transistor 13. Nmos transistor 13 is coupled between a constant current source 21 and ground with its drain 18 connected to the output of current source 21 and to output node 17. As V.sub.IN is varied, the voltage drop from source 19 to drain 18 responds by varying 180.degree. out of phase with V.sub.IN and having an amplitude gain determined by the architectural characteristics of transistors 13 and by the load line of amplifier 11. The load line of amplifier 11 is determined by the load at drain 18 and the voltage value of Vcc, which is typically 3V to 5V. One generally has no control over power supply Vcc variations, nor can one generally vary the architectural characteristics of transistor 13 after it has been manufactured. As shown, the only load coupled to drain 18 is current source 21. Thus, being able to select and maintain an accurate current value for current source 21 is an important criteria in maintaining a stable, predetermined gain for amplifier 11.
FIG. 2 shows amplifier 11 with a typical implementation of a current source. In FIG. 2, the current source consists of a pmos transistor 23 having its source electrode 25 coupled to Vcc, its drain electrode 27 coupled to drain 18 of transistor 13 and its gate 26 coupled to a reference voltage V.sub.REF. Due to structural and layout constraints, input signal V.sub.IN will generally also be coupled to reference signal V.sub.REF via an intrinsic coupling capacitor 29. As will be explained below, this can degrade the performance of amplifier 11.
With reference to FIG. 3, an enhancement mode transistor, such as pmos transistor 23 is characterized by a source-to-drain current, I.sub.DS, versus source-to-drain voltage, V.sub.DS, curve 31. Typically, the I.sub.DS vs. V.sub.DS curves of pmos transistors have an opposite polarity as those of nmos transistors. For the sake of clarity, all references to I.sub.DS, V.sub.DS and V.sub.GS refer to their magnitudes only, and not to their polarity such that the following discussion applies equally to pmos and nmos devices.
At a given source-to-gate voltage, V.sub.GS, within the saturation region, variations .DELTA.i in the source-to-drain current I.sub.DS are relatively small over a larger change .DELTA.v in the source-to-drain voltage V.sub.DS. This I.sub.DS vs V.sub.GS behavior will be identified as the transistor action of a switch transistor in the remainder of this application. Since I.sub.DS current remains relatively stable over a large V.sub.DS range, an enhancement mode MOS transistor operating in the saturation region is known in the art as a good current source. The saturation current, as well as the saturation mode of an MOS transistor, is selected by V.sub.GS. If V.sub.GS varies, the saturation current of transistors 23 will change, and transistor 23 may even fall out of saturation. Since the gain of amplifier 11 of FIG. 2 is dependent on a steady saturation current from transistor 23, it is important that reference voltage V.sub.REF, i.e. V.sub.GS in FIG. 3, be supplied by a constant voltage source.
With reference to FIG. 4, a good constant voltage source, such as a battery, experiences small voltage fluctuations .DELTA.v over a large current range .DELTA.i. As explained above in FIG. 3, the transistor action of a switch MOS device in its saturation region has the opposite characteristic of a large voltage fluctuation .DELTA.v over a small current change .DELTA.i. Therefore, this transistor action of an MOS transistor has traditionally not been suitable for generating a constant voltage source. A battery, however, is not available in an integrated circuit. One therefore is limited to transistors, resistors and other integratable devices when constructing a constant voltage source in an integrated circuit. In order to avoid the shortcomings of the transistor action discussed above, transistors are typically connected to function as diodes.
With reference to FIG. 5, a typical IC prior art circuit of a constant voltage source is shown. Transistor 24 is diode connected with its gate 22 coupled to its drain 28 such that its V.sub.GS is equal to its V.sub.DS. Diode connected transistor 24 is coupled in series with a current drain 35 between Vcc and ground. The reference voltage output, V.sub.REF, is tapped at node 38, which connects drain electrode 28 to current drain 35.
Line 39 of plot 37 illustrates the relationship between I.sub.DS and V.sub.GS of diode connected transistor 24. As shown, device 24 follows a more diode-like curve and current variations .DELTA.i result in less drastic voltage variations .DELTA.v than in the transistor action curve of FIG. 3. Diode connected transistor 24 thus has a more gradual relationship between its I.sub.DS current and V.sub.DS voltage.
Nonetheless, the use of diode connected transistors offers only a partial solution. As shown in plot 37, V.sub.DS is still highly susceptible to fluctuations in I.sub.DS, albeit to much lesser degree than before. A common method of reducing the susceptibility of V.sub.DS to I.sub.DS variations is to limit the amount of I.sub.DS current fluctuations .DELTA.i, and thereby limit V.sub.DS fluctuations .DELTA.v. Current fluctuations .DELTA.i are typically introduced by input signal V.sub.IN via coupling capacitor 29.
With reference to FIG. 6, current fluctuations .DELTA.i are traditionally limited by placing a large resistor 41 between node 38 and node 40, which connects to output signal V.sub.REF and coupling capacitor 29. The large resistance of resistor 41 reduces the amount of current introduced by V.sub.IN and thereby mitigates the amount of current fluctuations .DELTA.i through diode connected transistor 24. In order for resistor 41 to adequately reduce fluctuations in V.sub.REF, it needs to be very large and typically has a value of many megaohms. The formation of such large resistors in an integrated circuit requires a large area. Furthermore, large resistors in ICs suffer from various problems including leakage current and a distributed intrinsic capacitance of their own. Both problems introduce additional current fluctuations which reduce the resistor's effectiveness. Additionally, the circuit of FIG. 6 does not address voltage variations in V.sub.REF due to power fluctuations in Vcc.
Several attempts have been made to reduce this reliance on large resistors in the construction of IC constant voltage sources and high impedance nodes. U.S. Pat. No. 5,467,052 to Tsukada discloses a voltage reference generating circuit resistant to power fluctuations. Tsukada discloses the use of a first resistor in a first branch and a second resistor in a second branch, with the current through the second branch being a ratio of the two resistors and of the characteristics of some of the transistors used. Because the current is dependent on a ratio, smaller resistors may be used. In a similar approach, U.S. Pat. No. 4,264,874 to Young discloses two inter-coupled current mirrors with a resistor connected between one branch of the current mirrors and ground. U.S. Pat. No. 5,317,280 to Zimmer et al. discloses a method of creating a high impedance node using PFETs and multiple smaller resistors. Zimmer et al. use a bootstrap technique to multiply the resistance of a bias impedance by the ratio of two smaller resistors.
These approaches reduce the size of required resistors, but do not eliminate their use. It is possible to establish an integrated voltage source without the use of resistors by using only diode connected transistors, as shown in FIG. 5. Such circuits, however, are easily influenced by the introduction of error currents and Vcc fluctuations, as explained above.
It is an object of the present invention to provide a constant voltage source using only active devices and which is not affected by error currents introduced by an input signal or by Vcc fluctuations.
It is another objective of the present invention to provide a circuit for simulating a high impedance node without the use of resistors.
It is yet another objective of the present invention to provide a constant voltage source insensitive to power, temperature and input signal variations, having a high impedance node not requiring resistors, and being suitable for an IC circuit.